The development of compact, rugged, inexpensive optical spectrometers would benefit a range of applications, such as point-of-care medical devices, food analyzers, color sensors, field-installed chemical and environmental sensors, and personal environmental monitors. Today's most compact spectrometers utilize silicon photodetector arrays capped with interference bandpass filters; however, the high cost of fabrication of these devices make them commercially impractical.
At present, optical spectroscopy is utilized in many industrial and research settings, and the state of the art in small size and value are portable, textbook-sized units costing $3K including a PDA for monitoring the device (e.g. the SR2000, manufactured by Ocean Optics, Inc.) These units utilize a fiber input, diffractive optics, and a CCD detector mounted directly onto a PC-board, where the associated chips for signal-processing reside. While the resulting package represents a significant improvement in form factor and price-point over the alternative (table-top units costing $6K and up, e.g. the Oriel MS257 Spectrometer), these systems are still too large and expensive for use as ubiquitous sensors and personal monitors (e.g. environmental or medica).
There are currently a number of approaches being pursued in the development of micro-scale spectrometers, but each of the existing approaches has substantial drawbacks. Correia et al. (J. H. Correia et al, Sens. and Act., 82, 191-97 (2000)) report a single chip CMOS microspectrometer utilizing an array of Fabry-Perot etalons with different resonance cavity lengths. In this approach the fabrication is complex and expensive, and ill suited to even high cost applications. Yee et al. (G. M. Yee et al, Sens. and Act., 58, 61-66 (1997)) and Kung et al. (H. L. Kung et al, IEEE J. Sel. Top. Quant. Elec., 8, 98-105 (2002)) have demonstrated rudimentary spectrometers using micromachined components (a grating in the former, a mirror in the latter) and in both cases, fabrication complexity and expense once again limit the applicability of the approaches. A number of groups, such as L. Colace et al, Appl. Phys. Lett., 80, 3039-3041 (2002), have demonstrated so-called voltage-tunable photodetectors, for use as a sensing element in the wavelength stabilization of semiconductor lasers using feedback. While such devices could theoretically be used as the basis for a miniature spectrometer, again fabrication is difficult (involving a seven layer structure of InP and InGaAsP and micromachining) and expensive. Finally, Optical Coating Laboratory, Inc. has developed the manufacturing capability to produce a rectangular bragg reflector with a linearly varying pass band, with which they can construct a rugged and compact spectrometer (using an array of photodetectors) with good wavelength sensitivity and range. However, the bragg reflector strip, which consists of a multilayer dielectric stack with graded thicknesses, remains too expensive ($650 or more, depending on specifications) for low cost applications.
The present disclosure proposes a change in the detection paradigm that allows the use of silicon photodetectors capped with inexpensive, single-layer absorbing thin films. The present disclosure alleviates the need for expensive optical components (e.g., lenses and gratings) and intricate assembly during manufacturing, which are typical of existing spectrometer designs. At the same time, the ruggedness of the device is enhanced by vastly reducing the number of components and their complexity. The present disclosure describes a slim format spectrometer that would enable devices the size of handheld PDAs, for example, and ultimately smaller such as for a lab-on-a-chip application, and costing much less than the thousands of dollars of current devices, effectively rewriting the economics for field applications of optical spectroscopy. Furthermore, the active part of the spectrometer would be no larger that the detector array itself (e.g., 1 cm×1 cm×1 mm) enabling for the first time economical, small format, pervasive spectroscopy applications.